Gradual and selective trace-element enrichment in slab-released fluids at sub-arc depths

Area of Science:

• Geochemistry
• Mineralogy
• Petrology

Background:

• Subduction zone geochemistry is controlled by fluids and melts released from the subducting slab.
• Trace element transport by ultrahigh pressure (UHP) hydrous-silicate melts is known, but less so for UHP aqueous fluids.
• Understanding these agents is crucial for interpreting magma origins at convergent margins.

Purpose of the Study:

• Investigate trace-element enrichment and fractionation in UHP (>3.5-4 GPa) aqueous fluids.
• Compare the behavior of aqueous fluids with hydrous-silicate melts in trace-element transport.
• Determine the influence of these fluids on the geochemical signature of resulting magmas.

Main Methods:

• In situ LA-ICP-MS trace-element analyses of UHP fluid inclusions.
• Analysis of kyanite-bearing quartzite from the Sulu region (China).
• Examination of fluid inclusions trapped at varying prograde-to-peak metamorphic conditions.

Main Results:

• UHP aqueous fluids are enriched in LILE, U, Th, Sr, and REE.
• Progressive dissolution of minerals (phengite, carbonate, allanite/monazite) drives selective trace-element enrichment.
• Aqueous fluids fractionate trace elements distinctly from hydrous-silicate melts at UHP.

Conclusions:

• UHP aqueous fluids play a key role in trace-element transport and fractionation during subduction.
• The geochemical signature of magmas can reflect the nature of UHP fluids or melts involved in mantle metasomatism.
• This study highlights the distinct role of solute-rich aqueous fluids in shaping magma geochemistry at convergent margins.